I think that a big part of it is the Southern accent of most country singers. Many people can't stand it-- that's why more polished mainstream country is more universally liked.
16 Horsepower
Jay Munly. I recommend 'Big Black Bull.'
Slim Cessna's Auto Club. They are, however, known more for their
live show. Try the first video-- it's one of the most frightening things I've ever seen, even if the sound recording is low-quality.
And, of course,
Johnny Cash.